[SI-LIST] Re: question about voltage dividers
- From: "Andrew Ingraham" <a.ingraham@xxxxxxxx>
- To: <si-list@xxxxxxxxxxxxx>
- Date: Wed, 4 Mar 2009 10:06:21 -0500
> Here is an example of a 10:1 voltage divider with an input resistance of
> 10M
> ohms. 9M ohm resistor on top and 1M ohm resistor on the bottom.
This is just like those passive oscilloscope probes we used to use, back
when our 'scopes were full of tubes and 100 MHz was a pretty good scope
bandwidth. Those scope probes were OK for audio and low RF frequencies, but
not much beyond. (And you should never trust the advertised bandwidth of
many of those 10 Megohm 10:1 probes! Especially with the long ground
cliplead they come with.)
These days most high speed scopes use a 50 ohm input rather than 1 Megohm at
the front panel, so those older probes wouldn't work anyway even if you
tried to use one. But you can make your own passive divider probe that has
better bandwidth than those older probes, using much smaller resistances
than 9M/1M, and some work quite well for today's bandwidths and risetimes,
even without compensating capacitors because of the smaller resistances.
(I think Doug Smith has some tips for rolling-your-own compensated one, at
his website www.emcesd.com .) Of course these lower resistance passive
divider probes also load down the circuit you are probing, a lot more than
the 10 Meg probes did.
Andy
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